1. Field of the Invention
The present invention relates to methods and compositions for treating subterranean well formations, and more specifically, to improved methods and compositions for reducing the viscosity of subterranean servicing fluids where the servicing fluid has been crosslinked to increase its viscosity.
2. Description of the Prior Art
A variety of viscosified servicing fluids are used in subterranean applications, such as drilling fluids, fracturing fluids and gravel delivery fluids. Oftentimes, after the viscosified fluid has performed its desired task, it is necessary to reduce the viscosity so that the servicing fluid can be removed from the formation.
Fracturing operations commonly employ viscosified fluids to suspend propping particles. Fracturing generally involves pumping a viscous fracturing fluid into a subterranean formation at sufficient hydraulic pressure to create one or more cracks or “fractures.” The fracturing fluid must generally be viscous enough to suspend proppant particles that are placed in the fractures to hold the fracture open once the hydraulic pressure is released. Once at least one fracture is created and the proppant is substantially in place, the viscosity of the fracturing fluid is reduced and it is removed from the formation.
Similarly, sand control operations, such as gravel packing, use viscosified transport fluids to suspend gravel particles for delivery to an area in a well bore with unconsolidated or weakly consolidated particulates. One common type of gravel packing operation involves placing a gravel pack screen in the well bore and packing the annulus between the screen and the well bore with gravel of a specific size designed to prevent the passage of formation sand. The gravel pack screen is generally a filter assembly used to retain the gravel placed during gravel pack operation. A wide range of sizes and screen configurations are available to suit the characteristics of the gravel pack sand used. Similarly, a wide range of sizes of gravel is available to suit the characteristics of the unconsolidated or poorly consolidated particulates in the subterranean formation. The resulting structure presents a barrier to migrating sand from the formation while still permitting fluid flow. When installing the gravel pack, the gravel is carried to the formation in the form of a slurry by mixing the gravel with a transport fluid. Gravel packs act, inter alia, to stabilize the formation while causing minimal impairment to well productivity. The gravel, inter alia, acts to prevent the particulates from occluding the screen or migrating with the produced fluids, and the screen, inter alia, acts to prevent the gravel from entering the production tubing. Once the gravel pack is substantially in place, the viscosity of the transport fluid is reduced to allow it to be efficiently removed from the well bore.
Often, the viscosity of a servicing fluid is related to that fluid's pH. Thus, viscosity-reducing agents that reduce the pH of the servicing fluid may be added to reduce the viscosity of the fluid. To achieve that goal, gelled and cross-linked servicing fluids typically include internal delayed viscosity reducers such enzyme, oxidizing, acid, or temperature-activated viscosity reducers. However, these viscosity reducers may result in incomplete or premature viscosity reduction. Premature viscosity reduction can decrease the number and/or length of the fractures, and thus, can decrease the sought-after production-enhancing effects. Similarly, premature viscosity reduction of a gravel pack transport fluid may result in improper placement of the pack or insufficient packing of the gravel.
As an alternative to adding a viscosity-reducing agent to the servicing fluid, viscosity reduction may also be accomplished under the effect of time and temperature, as the viscosity of most servicing fluids will reduce naturally if given enough time and at a sufficient temperature. However, it is highly desirable to return the well back to production as quickly as possible and waiting for the viscosity of a servicing fluid to naturally decrease over time is generally unacceptable.